1. Field of the Invention
The present invention relates to the preparation of esters of .beta.,.gamma.-unsaturated carboxylic acids via the carbonylation of dienes, and, more especially, via the carbonylation of conjugated dienes with carbon monoxide in the presence of alcohol.
2. Description of the Prior Art
It is known to this art, e.g., from Japanese Pat. No. 48-5564, to prepare esters of .beta.,.gamma.-unsaturated carboxylic acids by carbonylation of conjugated dienes with carbon monoxide in the presence of a monoalcohol, a halide-free palladium catalyst and a halogen hydracid, at a temperature on the order of 100.degree. C. and under a carbon monoxide pressure on the order of 100 bars.
Unless a large amount of the catalyst and a high concentration of the halogen hydracid are used, this patented process does not simultaneously provide a high selectivity in the esters desired, a high conversion of the conjugated dienes and a good stability of the palladium catalyst.
It has also been proposed to prepare .beta.,.gamma.-unsaturated carboxylic acids or their esters by the carbonylation of conjugated dienes with carbon monoxide in an acid medium, in the presence of water or an alcohol, a palladium catalyst deposited upon suitable support and an organic phosphine. Compare French Pat. No. 1,461,826. This process displays the disadvantage of requiring high carbon monoxide pressures, for example, on the order of 700 bars.
U.S. Pat. No. 4,172,087 discloses the synthesis of penten-3-oic acid esters in admixture with a large amount of nonadien-3,8-oic acid esters, by the carbonylation of butadiene with carbon monoxide in the presence of an alcohol, an N-heterocyclic tertiary amine, and a palladium halide catalyst complexed with a monodentate tertiary phosphine, or a halide-free palladium salt complexed with a multidentate tertiary phosphine.
Such a process has very little selectivity as regards the esters penten-3-oic acid. Cf. U.S. Pat. No. 3,657,368.